If the keyword is a generic creature type that heroic-tier characters encounter often (goblin), it's common; specific creature type keywords (vampire) get bumped up to uncommon. Really weird creature types get pushed to rare.

If the keyword is a creature type from the Far Realms or Outer Planes (angel, githyanki) or is immortal (exalted, primordial), it's rare.

Judgment in between.

Other Characteristics

You can look at other creature characteristics for determining rarity.

Vision: a creature with darkvision is probably uncommon and a creature with truesight is probably rare.

Role: minions, soldiers, and skirmishers are probably common; artillery and brutes are probably uncommon; controllers and lurkers are probably rare. Same for Group Role: standard and minion are common; conjured and elite are uncommon; solo are rare.

Your Campaign

There are many ways to divide creatures into common, uncommon, and rare types, but the most important guideline is to do what is right for your game setting. What kinds of creatures do you want PCs to encounter most often? What kinds of creatures are "special treats" that show up very rarely?

Managing Rarity

So now you've decided what is rare and what is common. How do you manage this?

Make a list of common monsters. You don't need to make an exhaustive list. Just pick 10-20 that appear often in your campaign. Make another, shorter list of 3-5 uncommon monsters that you might want to use some time. Don't worry about rare monsters. Just pick them when you need one.

Random encounter tables. A good way to turn your decisions into something with mechanical weight is to create random encounter tables for different environments (forest, dungeon, urban, sewers, long roads, etc.). Make a 3d6 table for each one with the following bands, and then fill in some monsters. When you need a random encounter, roll and trust the dice.

Annotate your monster manuals. Sacrilege, right? You can buy some sheets of those adhesive "dots" in three different colors (I suggest green, yellow, and red for common, uncommon, and rare) and go through your books and put a dot next to each creature listing to indicate rarity. Of course, if you change your mind or start a new campaign with different goals, you will have to figure out how to get the dots off...

Make exhaustive lists. If you're a real masochist, find a list of all the creatures from the sources you use, and import them into a spreadsheet and add a "rarity" column and start categorizing...

If you want to reach back into the origins of the current crop of monsters, the Monster Manual from 1st and 2nd edition AD&D contained explicit "frequency" ratings of Common, Uncommon, Rare, and Very Rare.

I just looked in the d20 SRD and was shocked that this information wasn't there. Being an old-timey DM, this sort of information is just ingrained in me, so I never realised that newer DMs are deprived of this useful game tool. I was hoping I could point you at the SRD as a reference, but instead I'll can only suggest picking up a used copy of one of the old Monster Manuals if you get the chance. (It's a shame WotC doesn't sell the old edition PDFs anymore.)

Unfortunately, there are enough monsters that Wizards of the Coast have added to the game that weren't there originally, so the frequency statistics from the old Monster Manuals will only help with creatures that were in the game since the TSR days.